This invention relates to a control system for a vehicle safety device such as an air bag.
As disclosed in Japanese Laid-Open (Kokai) Patent Application No. 55031/74 and Japanese Laid-Open Utility Model Application No. 5371/90, there is known a system for controlling a vehicle safety device such as an air bag. This control system comprises an acceleration sensing circuit for outputting voltage corresponding to the acceleration of the vehicle, an amplifier circuit for amplifying the output from the acceleration sensing circuit, integrating means for integrating the output from the amplifier circuit, collision judgment means for comparing the integral value from the integrating means with a threshold level so as to judge whether or not a vehicle collision has occurred, and a drive circuit responsive to a collision judgment signal from the collision judgment means so as to supply electric current to thereby expand the air bag.
In the above control system, for example, when a large noise is applied to the acceleration sensing circuit, the integral value temporarily increases and may go beyond the threshold level. In such a case, despite the fact that any collision has not occurred, it is possible that the collision judgment means may decide the collision with the result that the air bag is inadvertently activated.
U.S. Pat. No. 4,836,024 constituting the prior art of the present invention discloses a control system provided with two acceleration sensors. The outputs of the two acceleration sensors are in phase with each other with respect to the acceleration and deceleration in the direction of running of the vehicle.
Two of the inventors of the present invention, together with another inventor, filed Jan. 18, 1991 a U.S. Pat. Application Ser. No. 642,951, and also filed Feb. 6, 1991 a U.S. Patent Application, both of which are directed to an air bag control system. These control systems are provided with two acceleration sensing circuits whose outputs are in phase with each other with respect to the acceleration and deceleration in the direction of running of the vehicle. Based on the output level of each of the acceleration sensing circuits, a microcomputer detects its malfunction.
An acceleration sensor disclosed in Japanese Laid-Open Patent Application No. 30693/80 is similar in construction to an acceleration sensor used in one embodiment of the present invention.